Exhaust-gas turbochargers can be used to increase the performance of internal combustion engines (for example, reciprocating-piston engines). An exhaust-gas turbocharger includes an exhaust-gas turbine in an exhaust-gas flow of the internal combustion engine and a compressor in the intake section of the internal combustion engine. A turbine wheel of the exhaust-gas turbine can be set in rotation by the exhaust-gas flow of the internal combustion engine and drives a rotor of the compressor via a shaft. The compressor increases the pressure in the intake section of the internal combustion engine, such that a greater quantity of air can pass into the combustion chambers during an intake. Exhaust-gas turbines can also be used as power turbines. In this case, they can drive via the shaft not the compressor of an exhaust-gas turbocharger but rather a generator or, via a clutch, some other mechanical power part.
Recent developments in the field of modern reciprocating-piston engines have been driven by a desire to reduce emissions, costs and fuel consumption. Here, the supercharging system of the engine can make a contribution to achieving these development aims. In the past, in large engines, use was made predominantly of exhaust-gas turbochargers with turbine and compressor components with fixed geometries. The geometries were designed and adapted for each individual engine. They were however invariable during the operating of the engine. To enable a better adaptation of the exhaust-gas turbocharger to the engine during operation in future, consideration is increasingly being given to the use of turbine geometries which can be adjusted (or varied) during operation (variable turbine geometries, VTG). Here, the opening of the guide blades of a guide device of the exhaust-gas turbine can be varied by a rotation of the guide blades. The use of adjustable turbine geometries is known in the field of small engines, as used, for example, in passenger motor vehicles. In large gas engines, variable turbine geometries are used which require precise regulation of the fuel/air ratio.
The flow components of the turbocharger have, for reasons of efficiency, been developed for high specific throughputs (i.e., high mass flow in relation to geometric size). The moving blades of the turbines of such turbomachines can be subjected to extreme vibration excitation. To ensure reliable operating behavior, precise coordination of the guide device (nozzle ring) and guide blade geometry is desirable in the development of the turbine.
In particular, a problem can arise that the guide blades of the guide device constitute a periodic disturbance for the moving blades of the turbine wheel, with a frequency equal to number of guide blades multiplied by rotational speed. If the frequency corresponds with natural frequencies of the moving blades, resonances can occur. The alternating stresses at the resonances can lead to material damage. It is known that the resonance amplitudes increase with decreasing opening of the guide blades. This can lead to a limitation of the admissible openings of the guide blades. In the variable turbine geometry, it is desirable to have a large available adjustment range of the guide blade opening. If the range of the admissible guide blade openings must be restricted as a result of inadmissible resonances, the benefit of the variable turbine geometry can be reduced.
From “Theoretical and Experimental Analysis of the Reduction of Rotor Blade Vibration in Turbomachinery Through the use of Modified Stator Vane Spacing”, R. H. Kemp, M. H. Hirschberg, W. C. Morgan. NACA Technical Note 4373, 1958, it is known that a non-uniform distribution of the circumferential position of the guide blades can bring about a considerable reduction in the resonance amplitudes. The non-uniform arrangement of the guide blades can be used in many turbomachines in order to reduce resonance amplitudes.
In exhaust-gas turbochargers for large engines, the variable guide device (VTG) can be constructed as a separate module and fastened to the gas inlet and gas outlet housings of the exhaust-gas turbine, as indicated in FIG. 1. An exhaust-gas turbine having a variable guide device fastened in this way is known from DE 100 13 335. The gas inlet housing and the gas outlet housing can generally be freely rotated in steps of defined angles, for example 15°, in order to be fitted to different engines. This can lead to the use of screws distributed uniformly over the circumference for example, in the case of segments of 15°, this results in 24 screws. If non-uniformly arranged guide blades are used, collisions between the guide blades and the screws are inevitable.
An exhaust-gas turbine having a variable guide device is likewise known from U.S. Pat. No. 3,542,484.